Microstructure and Crystallography of a Carbide-Free Bainite Steel under the Effect of Stress

Stress-affected bainite transformations were conducted on a Gleeble 3800 simulator, and the effects of different stress states (compressive, tensile, elastic, and plastic stresses) on the microstructural evolution and crystallography of a carbide-free bainite steel were investigated by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), and electron backscattering diffraction (EBSD). The results show that both compressive and tensile elastic stresses increase the thickness of bainite plates due to the additional mechanical driving force induced by the applied stress, and there is no significant difference between the effects of the two types of stress. Although prior deformation was reported to refine the bainite plates, the plastic stress, which induced prior deformation, coarsens the bainite plates because the effect of mechanical driving is more significant. The size of blocky martensite is refined and the length of bainite sheaves increases under the effect of stress, while the size of blocky RA is almost unaffected. In addition, EBSD results show that the orientation relationship between austenite and bainite ferrite is closer to the Nishiyama–Wassermann (N–W) relationship than to the Kurdjumov–Sachs (K–S) relationship for the stress-affected specimens. Strong variant selection occurs when a stress is applied during bainite transformation. Moreover, the elastic stress slightly decreases the hardness, while the plastic stress increased the hardness.